The classical picture of star formation states that stars form from the collapse of a gas molecular cloud. The star is born at the center of the cloud and the surrounding material forms a disc which rotates around a single young star. Studying the evolution of this kind of discs, mainly made of gas plus a little percentage of solids, is of key importance to understand how planets form. In other words, planets form within these protoplanetary discs made from the leftovers of the stellar formation process.

However, nature tends to be notoriously more complex than this simplistic picture. Stars do not form in isolation and, as a matter of fact, tend to live in groups called clusters. Such clusters can have different properties and stellar distributions. One thing is sure though: stars tend to live in multiple stellar systems, more so when they are young. These are systems where several stars orbit around each other and where protoplanetary discs can either form around a single star or around two or three of them. So, planetary cradles are clearly affected by the presence of several stars in the field. This is expected to have a dramatic impact on the way planets form and evolve within multiple stellar systems. The dynamical evolution of such systems is an active topic of research at the moment, both from the theoretical and observational points of view.

One aspect of particular interest is that during their infancy, stars occasionally experience close encounters with each other. This means that during their evolution a wanderer star could flyby close to other young stars harbouring discs. Such encounters occur somewhat randomly but can lead to strong perturbations that modify disc structure and hence its evolution. In fact, if the star moves in the same rotation sense as the disc, then the perturber star is able to produce very prominent spiral arms in the disc. These are clear signatures of an ongoing (prograde) stellar flyby, and are often called ‘stellar bridges’ since material spreads in between the two stars during the encounter (see image below).

However, as astronomers, we have very few direct observations of ongoing stellar encounters. Despite the very large number of young stars in the sky, these stellar ‘rendez-vous’ occur swiftly compared with the lifetime of discs and are therefore hard to catch. In our recent article (Dong, Liu, Cuello et al., Nature Astronomy, 2022) we discovered a point source at roughly 4600 astronomical units (au) from the binary protostar Z CMa. At first, one could think that it is simply a star that happens to be close to Z CMa – due to the projection on the sky plane – and that the newly discovered star is in reality much further away. But, upon close inspection, we realised that the disc around the binary Z CMa exhibited strong hints of an ongoing stellar flyby. In particular a very prominent ‘stellar bridge’ seems to connect the binary with the enigmatic point source at 4600 au.

The trick to solve this ‘stellar riddle’ was to combine different wavelengths (see image below). Simply put, we used different kinds of light to probe different layers or regions of the system. Specifically, we used two interferometers called Atacama Large Millimeter Array (ALMA) and Jansky Very Large Array (JVLA) in order to analyse the mm- and cm- wavelength emission coming from Z CMa. We detected both the gas and the dust that constitute the disc around the binary star. In addition, we confirmed the presence of a very prominent spiral in the circumbinary disc, which was previously reported in scattered light using the Subaru telescope (at infrared wavelengths). More importantly, thanks to ALMA and JVLA, we revealed a point-like source located along the spiral extension in the very outer regions of the disc: a disc configuration similar to the one expected shortly after a stellar encounter. In fact, the aftermath of a prograde stellar flyby is unequivocal: a strongly perturbed disc and a wanderer star leaving the field of view. This appeared to us as the most reasonable explanation for Z CMa, but a single snapshot as the one obtained through observations did not allow us to settle the matter.

To test our preferred scenario, we performed a series of hydrodynamical simulations to check whether or not a flyby-star could be responsible for the observed disc structure. After months of investigation and a long phase of trial-and-error, we found a flyby scenario in agreement with the observational constraints (see image below). Our study suggests that the wanderer star is  relatively light compared to the binary, passes at less than 2000 au from the outer edge of the circumbinary disc, and is on an inclined prograde orbit. Such a  perturber is able to trigger a misaligned prominent spiral – both in the gas and in the dust – seen as a bright ‘stellar bridge’ between the binary and the newly found point source.

We tested our hypothesis by  directly comparing models with real telescope observations. To do that, we ran radiative transfer calculations after hydrodynamics simulations where we lighted up the stars and computed the way the circumbinary disc reprocesses the stellar radiation. By doing so, we obtained synthetic images, and compared them with real observations (see image below). Our synthetic observations are in good agreement with observations, which convinced us  that the flyby scenario is the most robust dynamical process able to explain the circumbinary disc morphology.

However, there are still unanswered questions regarding the formation and evolution of Z CMa. For instance, why does the inner binary seem to be undergoing a spectacular accretion event? How is this connected to the flyby scenario? Why does the newly discovered star appear invisible at infrared wavelengths? And more generally, how did this multiple stellar system form? Z CMa is special in the sense that it belongs to the rare group of stars where a flyby was caught red-handed. These extreme events are expected to deeply modify disc dynamics, which unavoidably alter the process of planet formation within protoplanetary discs. Our study of Z CMa sheds light on how flybys shape planetary cradles in regions of active star formation and will certainly help to interpret future observations of similar systems.